Propagation steps in free radical substitution of methane

Steps:

• Initiation: Br2 → 2Br• (produces bromine radicals).
• First propagation: Br• + CH4 → HBr + CH3• (Br• reacts with methane to form methyl radical).
• Second propagation: CH3• + Br2 → CH3Br + Br• (CH3• reacts with Br2 to regenerate Br•).
• These steps propagate the chain, with Br• and CH3• as key radical reactants.

Why B is correct:

• In propagation, Br• and CH3• are the radicals that react to sustain the mechanism, matching the definition of propagation reactants.

Why the others are wrong:

• A: H is a product (HBr), not a propagation reactant; Br• is correct but paired wrongly.
• C: CH4 is a starting molecule in initiation-like step, but Br• + CH4 is propagation; option lists non-radicals.
• D: CH3• is a radical, but Br2 is not a propagation reactant—it's consumed in the second step.

Final answer: B